This invention relates generally to switches or relays and in particular to pressure actuated relays in which electrical switch contacts are caused to make and break in sequence in response to manually exerted pressure. More particularly, this invention relates to a pendant switching station provided for the operation of a hoist or the like.
There are innumerable industrial applications in which it is desirable to apply power simultaneously to two or more industrial components. A typical application is found in an electric hoist in which it is essential to apply electrical power to one or more coils of a hoist motor and at the same time to a coil that electrically actuates release of the brake. Accordingly, multiple contact switches have been developed to provide the application of power to a plurality of industrial components. One such switch is disclosed in U.S. Pat. No. 3,087,031. A difficulty with achieving switching simultaneity arises with switches of this kind and with switches of other prior art devices, however, as it is difficult to assure that the plurality of contacts all make and break at the same moment. The difficulty is aggravated by the fact that the make and break contacts tend to erode at different rates and may accumulate non-uniform deposits of dirt or other foreign matter. As a result, the plurality of electrical contacts not only exhibit different electrical resistances and therefore pass electrical currents of different magnitudes, but also may close their respective circuits at non-simultaneous moments.
It is well known that the erosion of electrical contacts and the problems caused by dirt accumulation can be minimized by the use of exotic and expensive rare earth alloys at the contacts rather than less expensive base metal alloys. As is readily apparent however, switches which have multiple contacts for the application of power to a plurality of devices can be rather expensive when each of the contacts include these rare earth alloys. In fact, the cost of the alloy contacts alone can be a major contribution to the overall cost of the multiple contact switch. It is also well known that electrical contacts passing larger power currents sometimes tend to "weld" together making it difficult, if not impossible, to break the circuit. This is obviously undesirable since a malfunctioning switch operating a device such as a hoist could cause a serious industrial accident if the hoist could not be turned off at the appropriate moment in either its lift or lower modes. These and other difficulties have created a need for an inexpensive, reliable, multi-contact switch which has a positive means for breaking the circuit.